When it becomes necessary to shift the fuel system from the main control to back-up control, as for example because of a malfunction in the main control, the transition may result in an excess of fuel if both controls are delivering fuel simultaneously or in a shortage of fuel if neither is supplying fuel. In either event, the result may be engine damage or failure. Where the fuel is supplied by a variable volume pump responsive to the pressure drop across the metering valve it is essential that the pressure signals reach the pump from the operative control during the transition from one control to the other as well as before and after the operation of the transfer valve by which the transition is made.